The freeze-etch method of preparation of specimens for electron microscopy allows one to visualize internal faces and external surfaces of biological membranes stabilized by rapid freezing. The faces and surfaces of nearly all types of membranes exhibit characteristic particulate components that are assumed to be globular membrane proteins, but such a correlation has so far only been made for certain glycoproteins of erythrocyte membranes (Pinto da Silva, Douglas and Branton, 1971, Marchesi, Tillack, Jackson, Segrest and Scott, 1972). Because of the widespread distribution and the potential importance of these particles for understanding membrane structure and function it is desirable to identify further membrane components and map their distribution on the exposed faces. Such information would provide us with a more precise knowledge of the architecture of membranes at the supramolecular level, and illustrate how specific enzymatic and structural membrane components are spatially arranged and interact to provide a membrane with certain properties. Work already in progress in this laboratory as well as various reports in the literature indicates that such structure-function correlations can be made by examining the structure and composition of carefully selected membrane systems with a combination of electron-microscopic, immunological and biochemical methods using normal and mutant membranes, and comparing the findings with known and measured properties of these membranes. To this end the following systems will be investigated: chloroplast membranes, tight junction membranes and ciliary membranes. In addition, experiments are outlined which should allow us to determine factors that regulate the assembly of membrane components into structures of higher order.